to them.
The first great discovery was that of the daily movement of the earth,
its rotation on its own axis, which makes it appear as if all these
shining things went round it. It is indeed a very difficult matter to
judge which of two objects is moving unless we can compare them both
with something outside. You must have noticed this when you are sitting
in a train at a station, and there is another train on the other side of
yours. For if one of the trains moves gently, either yours or the other,
you cannot tell which one it is unless you look at the station platform;
and if your position remains the same in regard to that, you know that
your train is still standing, while the other one beside it has begun to
move. And I am quite sure that there is no one of us who has not, at one
time or another, stood on a bridge and watched the water running away
underneath until we felt quite dizzy, and it seemed as if the water were
standing still and the bridge, with ourselves on it, was flying swiftly
away backwards. It is only when we turn to the banks and find them
standing still, that we realize the bridge is not moving, and that it is
the running water that makes it seem to do so. These everyday instances
show us how difficult it is to judge whether we are moving or an outside
object unless we have something else to compare with it. And the
marvellous truth is that, instead of the sun and moon and stars rolling
round the earth, it is the earth that is spinning round day by day,
while the sun and the stars are comparatively still; and, though the
moon does move, yet when we see her get up in the east and go down in
the west that is due to our own movement and not to hers.

The earth turns completely round once in a day and night. If you take an
orange and stick a knitting-needle through it, and hold it so that the
needle is not quite straight up but a little slanting, and then twirl it
round, you will get quite a good idea of the earth, though of course
there is no great pole like a gigantic needle stuck through it, that is
only to make it easy for you to hold it by. In spinning the orange you
are turning it as the earth turns day by day, or, as astronomers express
it, as it rotates on its axis.

There is a story of a cruel Eastern King who told a prisoner that he
must die if he did not answer three questions correctly, and the
questions were very difficult; this is one of them:

'How long would it take a man to go round